Unit 5: Age of the Earth
E5.3D Describe how index fossils can be used to determine time
E5.3g Identify a sequence of geologic events using relative-age
In the same way that a history book shows an order of events, layers of rock
(called strata) show the sequence of events that took place in the past. Using a
few basic principles, scientists can determine the order in which rock layers
formed. Once they can know the order, a relative age can be determined for each
*Relative age indicates that one layer is older or younger than another layer, but
does not indicate the rock’s age in years (absolute age).
Law of Superposition
Sedimentary rocks form when new
sediments are deposited on top of old
layers of sediment. As the sediments
accumulate, they are compressed and
harden into sedimentary rock layers.
Scientists use a basic principle called the
Law of Superposition to determine the
relative age of a layer of sedimentary
rock. *The Law of Superposition is
that an undeformed sedimentary rock
layer is older than the layers about it and
younger than the layers below it.
According to the Law of Superposition,
layer 1 was the first layer deposited, and
thus the oldest layer. The last layer
deposited was layer 12, and thus it is the
*Principle of Original
Scientists also know that sedimentary rock generally forms in
horizontal layers. The Principle of Original Horizontality is
that sedimentary rocks left undisturbed will remain in horizontal
layers. Therefore, scientists can assume that sedimentary rock
layers that are not horizontal have been tilted or deformed by
crustal movements that happened after the layers formed.
Movements of Earth’s crust can lift up rock layers
that were buried and expose them to erosion.
Then, if sediments are deposited, new rock layers
form in place of the eroded layers. The missing
rock layers create a break in the geologic record
in the same way that pages missing from a book
create a break in a story.
*A break in the geologic record is called an
unconformity. An unconformity shows that
deposition stopped for a period of time, and rock
may have been removed by erosion before
There are three types of unconformities.
1. An unconformity in which stratified (layers) of
rock rests upon unstratified rock is called a
2. The boundary between a set of tilted layers
and a set of horizontal layers is called an
3. The boundary between horizontal layers of old
sedimentary rock and younger, overlying
layers that are deposited on an eroded surface
is called a disconformity.
According to the Law of Superposition, all rocks beneath an unconformity are
older than the rocks above the unconformity.
When rock layers have been disturbed by faults (a break or crack in Earth’s crust)
or intrusions (a mass of igneous rock that forms when magma is injected into rock
and then cools and solidifies), determining relative age may be difficult. In such
cases, scientists may apply the Law of Crosscutting Relationships. *The Law
of Crosscutting Relationships is that a fault or intrusion is always younger
than all the rocks it cuts through above and below the unconformity.
Based on what you now know about the Law of
Superposition, the Principle of Original
Horizontality, unconformities, and the Law of
Crosscutting Relationships can you place the layers
indicated in the diagram in the correct order,
starting from the oldest layer?
The oldest layer is Q, followed by O, then N, then
M, then L. P cuts across layers L-Q, so it is the
next layer since it does not cut into layer H.
Above the unconformity we then have layer H,
followed by I, then J, with K being the youngest
Paleontologists can use fossils to determine the relative ages of the rock layers in
which the fossils are located. Fossils that occur only in rock layers of a particular
geologic age are called index fossils.
To be an index fossil, a fossil must meet certain requirements:
1. It must be present in rocks scattered over a large region.
2. It must have features that clearly distinguish it from other fossils.
3. Organisms from which the fossil formed must have lived during a short span of
4. The fossil must occur in fairly large numbers within the rock layers.